Incandescent lamp

ABSTRACT

Tungsten-halogen lamps of extended life are obtained by coating internal surfaces of the discharge tube and exposed surfaces of internal components with metal phosphate or arsenate glasses. The preferred glass is aluminium titanium phosphate. The coatings may be formed by applying a liquid composition capable of generating the desired phosphate or arsenate to the surfaces to be coated, and subsequently heating them to remove the liquid medium and form a vitreous coating. The medium may be water or an organic solvent such as an alcohol, and may contain a dissolved compound of the metal and an oxyacid of phosphorus or arsenic.

United States Patent Mason et al.

l l [NCANDESCENT LAMP Jan. 18. 1974 [Y 3] Assigneei {22] Filed:

Appl. No. 434,38l

{3U} Foreign Application Priority Data .lnn I), I97. limtcd Kingdom2951373 ISZl lj.S.(TI.. 313/221: 3l3/2I2; ll7/2ll [5 l l lnt. (l.. HtlljGU06; Hill hl/Zt) [58, Field of Search .3l3/Z2l. 212. 2H4; ll7/2l l, ZUI

[Fol References Cited Aug. 26, 1975 2.393.469 H1946 Hoolcy 4 i i i i v vv .i 3l3/22l X (57] ABSTRACT Tungsten-halogen lamps of extended life areobtained by coating internal surfaces of the discharge tube and exposedsurfaces of internal components with metal phosphate or arsenateglasses, The preferred glass is aluminium titanium phosphatev Thecoatings may he formed by applying a liquid composition capable ofgenerating the desired phosphate or arsenatc to the surfaces to becoated and subsequently heating them to remove the liquid medium andform a vitreous coating. The medium may be water or an organic solventsuch as an alcohol and may contain a dissolved compound of the metal andan oxyacid of phosphorus or arsenic 6 Claims, 1 Drawing FigureINCANDESCENT LAMP This invention relates to electric incandescent lamps,and more especially to lamps operating by a tungstenhalogen cycle.

In any incandescent tungsten filament lamp containing a reactive fillsuch as halogen or halide, the choice of material for the internalcomponents and envelope is usually very restricted. For lamps havingiodine. bro mine or chlorine in the fill the envelope is preferablyfused quartz or a high silica content glass and the lead in wire,filament supports, internal reflectors, shields and other internalcomponents are substantially composed of molybdenum or tungsten. if lessexpensive, common materials such as nickel, iron, copper, aluminum andalloys containing these are used they react with the halogens to formhalides which can cause filament embrittlement, and/or a halogendeficiency, both resulting in severely reduced filament life. Also, ifsoft glass. such as soda lime silicate, is used for the envelope. apartfrom the obvious difficulties of the low softening temperature and highwater content, the alkali metals can react with the halogen or halides,again reducing filament life.

In operation, tungstenhalogen lamps normally contain a non-reactive gasfilling such as N Ar, Kr or Xe together with iodine, bromine or chlorinevapour which combines with the evaporated tungsten escaping from theincandescent filament. An equilibrium concentration is attained by thegaseous species within the lamp between the temperature limits definedby the incandescent filament and coldest spot on the lamp envelope. Thecold spot temperature must be sufficiently high to prevent any tungstenhalide from condensing, and provided that this condition is met acontinuous tungsten transport cycle operates which keeps the envelopefree from tungsten. The minimum envelope temperature depends upon thehalogen or halogens taking part in the cycle. However, the maximumenvelope temperature is usually well above the acceptable limit for softglass, and for this reason tungsten-halogen lamp envelopes are usuallymade from vitreous fused silica on high silica content glasses.

The return of tungsten to the filament does not in itself increasefilament life since tungsten iodides, bromides and chlorides dissociatewell below normal filament operating temperatures. Radio-chemicaltracers have shown that evaporated tungsten is redistributed during thelife of the lamp so that the cooler parts of the filament collecttungsten at a greater rate than the hotter parts. Filament failureusually occurs quite normally by the subsequent burn-out of a hot spot.The improvement in life of tungsten-halogen lamps in comparison withconventional incandescent lamps is for quite a different reason. Theabsence of envelope blackening coupled with the requirement for awell-defined mini mum envelope temperature dictates that the envelopemust be substantially smaller than that of a conventional counterpart.In fact, tungsten-halogen lamp en' velopes are usually small andmechanically strong and in consequence can be safely gas-filled toseveral atmospheres prcssure. This increased gas filling pressureaccounts for the gain in life.

If filament hot spots could be healed or prevented a further extensionin filament life would be possible. This is feasible with atungsten-fluorine transport cycle because in this case the most stabletungsten fluoride dissociates at a temperature above 3()()(lC and tungsten is returned to the incandescent filament surface. Again, this hasbeen substantiated by radiochemical tracer experiments, which show thattungsten vapour returned from the region of the envelope is evenlydistributed along the incandescent part of the filament. Technologicaldifficulties have prevented the further development of tungstcn-fluorinelamps, the principal problem being that free fluorine reacts rapidlywith solid tungsten below about 200()C, the cold parts of the filament,the lead wires and the supports being rapidly eroded, and that thefluorides formed (e.g. tungsten fluorides) react with the silicacontained in the envelope material to form SiF depositing tungsten onthe tube wall. This uses up die free fluorine in a very short time.Various methods have been proposed for protecting the envelope andtungsten components but these have been unsuccessful because of theinability to produce a continuous thin layer of protective material freefrom pin-holes and minor defects.

The present invention seeks to provide a protective layer for theexposed internal surfaces of incandescent lamps which tend to react withthe fill in the lamp envelope, particularly where this includes ahalogen, and more especially fluorine or a fluorine-containing compound.

ln lamps according to this invention, at least those portions of theinternal surface of the envelope and the exposed surfaces ofinternalcomponents which tend to react with the fill in the envelope duringoperation of the lamp are provided with a coating of a metal phosphateor arsenate glass composition. The surfaces to be covered may includethe internal surface of the envelope, the filament tails or lead-inwires or the filament supports, depending on the nature of the fill gasemployed and on the materials from which the envelope and the internalcomponents are fabricated. Part or all of the filament or filaments maybe initially provided with a coating according to the invention, forexample where the coating technique cannot conveniently avoid this, butthe coating on the filament will be removed when the filament is heatedto incandescence.

The protective coatings provided in accordance with this invention maybe applied to conventional materials used for the fabrication of lampcomponents, for example to protect them from highly reactive fillsubstances, or they may enable cheaper and more readily availablematerials to be substituted for conventionally used materials withoutunacceptable loss in performance or life.

The coating is preferably derived from an aluminium phosphate complex asdescribed in German Offen legungschrift (DOS) No. 2,028,839 (BritishPat. Nos. 1,322,722 and 1,322,729) or from one or more of the metalphosphate or arsenate compositions prepared in accordance with DOS No.2,235.65l or from a eomposition comprising an aluminium phosphate andcontain ing a titanium compound prepared in accordance with DOS No.2,35l,954, the latter being at present most preferred. Combinations ofthese compositions can also be used.

For the purposes of this invention, preferred metal phosphates andarsenates are those of atomic number l2 to I4, 20 to 32, 39 to 50, 5b to8G, or 92. The term phosphate is here meant to include ortho-. metaandpyro-phosphates together with phosphinates and phosphonates.

Especially preferred sources of metal phosphate coatings aresolventsoluble complex phosphates containing co-ordinated solventgroups, such as water or polar organic solvents, as described in DOS No.2,028,839 and 2,235,65l. Not only are the isolated complex phosphatesthemselves suitable, but the compositions which are therein describedcontaining phosphate precursors may also be used.

Liquid coating compositions may be used which comprise a solution, of(a) a metal compound and (b) an oxyacid of phosphorus or arsenic, or acompound capable of forming such an oxyacid in the solution. At leastpart of the solvent may be organic. These compositions are capable ofdecomposing to a metal phosphate or arsenate on being heated.

The solvent is selected from water or the wide range of organic solventswhich dissolve the components of the composition. The organic solvent,when used, is preferably selected from alcohols, esters, ketones,aldehycles, nitrocompounds and ethers, especially monohydric alcohols ofthe structure ROI-l, esters of the structure RCOOR, ethers of thestructure ROR ketones of the structure RCOR nitrocompounds of thestructure RNO; and ethers of the structure OR, where R, R and R arealkyl groups or substituted alkyl groups containing from l to carbonatoms each, and R is a divalent alkyl group having from 4 to 7 carbonatoms one of which may be replaced by an oxygen atom. Mixtures of one ormore solvents may be used. Diluents may also be present, provided theydo not bring about precipitation of the components of the composition.

Aliphatic alcohols containing 1 to ID carbon atoms are particularlyconvenient, especially lower molecular weight alcohols containing 1 to 4carbon atoms, for example methanol, ethanol, nor iso-propanol andsubstituted alcohols especially methoxyor ethoxy-ethanol. Suitableesters are ethyl acetate or carbonate. Acetyl acetone may be used.Tetrahydrofuran is the most preferred ether to use, though dioxan mayalso be used. Aromatic hydroxy compounds can be used, but solubility islow in such materials.

The composition may be formed by dissolving an isolated complex of thetype described in the specifications referred to above in a solvent. Themetal compound may itself be a phosphate and so provide the oxyacid ofphosphorus or arsenic, in which case an additional acid may be requiredto form a homogeneous solution, e.g. hydrochloric or nitric acid.

A wide range of metal compounds may be used. Simple inorganic compoundsincluding oxides and hydroxides are suitable, as are salts such ashalides, carbonates, nitrates, phosphatcs, perchloratcs and cyanates.Sulphates may be used in some cases but they can be disadvantageousowing to the difficulty with which they are thermally decomposed.

Also suitable are salts of organic acids such as acetates, bcnzoates,oxalates, propionatcs or formates. Alkoxidcs are also useful.

Alternatively, co-ordination complexes of the metal may be used, forexample complexes having ligands derived from acctylacetone,ethylcnedithiol, ethanol amine, carbon monoxide or phosphines.

Preferred compositions are those in which the metal and oxyacid arepresent with atomic ratios of metal to phosphorus or arsenic from l:().!to l:2.9. Preferred metals are aluminium, iron, chromium, titaniumvanadium and tin. Outstanding results have been achieved with thisinvention using compositions containing both aluminium and titanium.

A solvent-soluble aluminium phosphate may be used, for example the acidorthophosphatcs Al (HPO and Al(H PO,);,, and mixtures containing them.

Normal aluminium orthophosphate is insoluble in water but soluble indilute mineral acids (for cxampe hydrochloric and nitric acids) and insome carboxylic acids (for example citric acid) and such solutions maybe used for the purpose of this invention. Moreover, solid complexaluminium phosphates containing the anion of the acid andchemically-bound water or alcohol (or a mixture thereof) may also beused.

Where the complex contains an alcohol gx'up, it is preferred that it bean aliphatic alcohol containing from one to four carbon atoms, forexample methyl alcohol, ethyl alcohol, n-propyl alcohol or isopropylalcohol, although complexes with higher alcohols are known and may beused if desired.

The complex phosphates most commonly contain from three to fivemolecules of the hydroxy compound per phosphorus atom, for examplewater-containing complexes may have an empirical formula correspondingto AlPO,,.HCl.(H O), where x is in the range 3 to 5.

The complex aluminium phosphates containing alcohol and their solutionsmay be prepared by reacting aluminium compound, preferably halide, withan alcohol and phosphoric acid. One such compound has the empiricalformula Al P Cl H C O The complex phosphate containing water can be madeas above or by hydrolysing the alcoholcontaining complex phosphates or,for example, by contacting aluminium phosphate hydrate with gaseoushydrogen chloride.

Iron, chromium, vanadium tin and titanium phosphate containing coatingsmay be prepared by dissolving a salt, preferably a halide, in an alcoholand adding phosphoric acid or a source thereof.

The glass layer should be free from pin-holes or other defect orimperfection which might cause it to break down during operation of thelamp. In one preferred method of making lamps according to thisinvention, the desired portions of the internal surface of the envelopeand the surfaces of internal components which are exposed in thefinished lamp are coated either separately or after assembly with aliquid composition capable of generating the desired metal phosphate orarsenate, and subsequently heated to evaporate the solvent and cure thecomposition to form a defectfree metal phosphate or arsenatc coating. Ithas been found valuable in the production of defect-free coatings toallow the applied liquid coating composition to drain thoroughly andthereafter to bake initially at a relatively low temperature to removethe solvent and subsequently at a controlled higher temperature tocomplete the formation of the protective coating. The preferred bakingtemperatures vary with the particular composi tion of coating materialemployed, but can be determined by experiment,

One example of this technique, will now be described with reference tothe accompanying drawing, which shows diagrammatically atungsten-halogen lamp assembly in the course of manufacture.

As shown in the drawing, a 12V. W. tungstenhalogen lamp, of the typecommonly used in projector and motor vehicle lighting applications,comprises a fused quartz envelope 1 in which is sealed a tungstenfilament 2 supported on filament tails or lead-in wires 3 and isprovided with an exhaust tube 4. The lamp is to be provided with a metalphosphate glass barrier layer covering the inside surface of theenvelope 1, the filament 2 and filament tails 3.

A liquid coating composition containing the metal phosphate or arscnateis dispensed from a hypodermic syringe through the lamp exhaust tube 4by inserting the needle of the syringe. discharging the liquidcomposition and then almost immediately drawing it back into thesyringe, leaving only a thin layer adhering to the inside surfaces ofthe lamp structure. At this stage the lamp is inverted to drain, andthen heated in a vacuum or suitably inert atmosphere. for example atapproximately 100C for an hour in the case of a methanolic composition.The metal phosphate glass coating is finally formed by baking at ahigher temperature. for example at 3(l()5()0C in a vacuum or suitablyinert atmosphere for about three minutes in the case of an aluminiumtitanium phosphate composition. The final bake can be effectivelyincorporated in subsequent lamp processing.

The initial heating cycle is chosen to substantially re move the solventand the time. temperature and atmosphere will depend upon the solventselected. The temperature of the subsequent bake depends on theparticular formulation used, but will in general be below l()O()C.

The lamp is then processed in the normal manner for tungsten-halogenlamps. When the filament is first energised the metal phosphate glasslayer on the incandescent filament surface and part of the filament tailadjacent to the filament is removed. leaving a protective barrier 5 onthe envelope surface and cold parts of the filament tails or lead-inwires as shown.

In accordance with one aspect of this invention it has been found thatwhen such lamps are provided with a fluorine-containing fill they can beoperated with less or even substantially no attack on the filamenttails. the filament or the envelope surface by fluorine or fluorides.The fluorine can be added as the element, or more conveniently as WFwithin the pressure range of l to Torr. or as NF or a solid such as NFSbF NlflAsF. XeF.SbF,; XelflAsF... TeF ,SbF or SeF,SbF Solids may alsobe added in solution in suitable solvents as disclosed in thespecification of our British Pat. No. l236l74.

In accordance with another aspect of this invention, cheaper or moreeasily obtainable or workable materials are used for the envelope orinternal components of tungstemhalogen lamps by providing on the exposedsurfaces of such parts of the structure a coating of a metal phosphateor arscnate glass as described above.

In certain established tungsten-halogen lamps (e.g. twin filament carlamps) a molybdenum frame or wires is or are used both as lead-inconductors and as a member to carry a molybdenum (or tungsten) shield.There is some evidence to suggest that there is a limited chemicalreaction between these components and the fill. and in such a case it isadvantageous to coat them with a halogenor halide-resistant layer of thephosphate or arsenate glass. However, as an alternative, the refractorymetal in these components can be replaced by a less expensive and easierto work metal. such as iron or nickel, coated with one of theaforementioned glasses.

A further possibility is to use a glass envelope coated with a halogenorhalide-resistant layer of phosphate or arsenate glass in place of thefused quartz conventionally employed for such envelopes. This mayinvolve a direct replacement of fused quartz by a hard glass. such asborosilicate or aluminosilicate, or the use of inexpensive sodalimesilicate soft glass. In the latter case the envelope dimensions shouldbe carefully chosen so that the hottest part is below the glass straintemperature and the coldest part is above the well-established minimumfor the particular tungsten-halogen cycle to function. This also wouldreduce material and manufacturing costs. It should be noted thataluminosilicate glass is used for the envelope material of certaintungsten-halogen lamps but cannot be considered as a replacement forfused quartz. It will thus be apparent that individual components or allthe internal surfaces within the lamp may be coated.

The following are specific examples of the practical application of thepresent invention and in the production of tungsten-fluorine lamps.

EXAMPLE I A liquid aluminium titanium phosphate coating composition wasprepared by dissolving anhydrous aluminium chloride (1.946 g.) inmethanol (992.467 g.) and pouring the solution into titaniumtetrachloride (3.96l g.). Orthophosphoric acid L626 g.) was added to theresultant solution.

A tungsten filament lamp assembly was coated internally with thiscomposition by the technique described above and the coated assemblythoroughly drained, heated at l00C in vacuo for l hour, and baked at400C for 3 minutes also in vacuo. The lamp was subsequently filled with3% atm. argon and 4 Torr WF and finished in the usual way.

In operation, the lamp was successfully run at a fila' ment temperatureof 3000C for 25 hours, and the failure at that time was not due to abreakdown of the coating. In contrast. similar lamps without the coatingof this invention showed extremely rapid loss of fluorine due toreaction with the lamp components and had a life which in no caseexceeded 2-3 minutes.

EXAMPLE 2 Lamps were made as described in Example I except that a gasfilling of 3V2 atm. pressure of nitrogen and 5 Torr of NF was used. Thereduced activity of this system. coupled with the protective coating,enabled lives of hours to be achieved and. again. the coating had notbroken down at the end of life.

EXAMPLE 3 A series of tungsten-fluorine lamps were prepared as describedin Example l but using the coating formula tions set out below and gasfills of 3% atm. argon and 4 Torr WF The coated assemblies were baked atl0llC in vacuo for 1 hour and subsequently at 2()06()()C for 3 minutesin vacuo.

The coating compositions were prepared from the following components ina similar manner to that described in Example l. all percentages beingby weight.

Content of aluminium phosphate complex with four methanol ligands If:calc.) 0.42 Content of titanium (L calc.)

Formulation A is identical in composition to that used in Example l,while B and C have respectively half and twice the titaniumconcentration.

With a final bake at 400C in vacuo all three formulations gave lampswith a life of about 25 hours. Variation of the baking temperature hadless effect in the case of formulation C (0.2% Ti) than in the othercases, and equally good results were obtained over the range 300 to500C.

instead of aluminium titanium phosphate compositions described in theabove preferred examples, aluminium phosphate coatings may be used,prepared from solutions of halogen-containing complex phos phates ofaluminium as disclosed in DOS No. 2,028,839, coating the internal lampsurfaces. and heating to cure the coating under the conditionssubstantially as disclosed in the same Application.

EXAMPLE 4 Tungsten-fluorine lamps were prepared as described in Example1 except that the coating compositions employed were aluminium phosphatecompositions (without titanium) as described in DOS No. 2,028,839.

Using the heating and baking conditions of Example 1, which areespecially suited to the aluminium tita nium phosphate composition, thelamps coated with aluminium phosphate were found to have a useful lifeof about minutes, after which time they showed evidence of loss offluorine and attack on the structure by fluorine and fluorides. Althoughthe coatings employed in this example gave the lamps much lessprotection than the coating of Example I, the life of the lamps wasnevertheless notably greater than the life of uncoated lamps.

Instead of one of the above compositions, coatings may be used preparedfrom liquid compositions of other metal compounds and oxyacids ofphosphorus or arsenic as disclosed in DOS No. 2,235,651 and the 8 otherApplications listed with it above, coating the internal lamp surfacesand heating under the conditions substantially as disclosed in the sameApplications, the remainder of the processing following the same generallines as in the above preferred examples.

It should be noted that it is not an essential part of the process ofthis invention to coat the envelope and internal components afterassembly, as described in the above preferred examples, and individualcomponents may be coated before lamp assembly. The essential feature ofthe invention is the provision of a continuous layer consistingessentially of a metal phosphate or arsenate glass covering the interiorsurface of the envelope or any internal components that could react withhalogen or tungsten halides at the lamp operating temperatures.

What we claim is:

l. A tungsten-halogen lamp comprising:

a light-transmitting envelope;

internal components including a tungsten filament and supports thereforsealed within said envelope; electrical leads for said filament sealedinto said en' velope;

a gaseous fill including halogen in said envelope;

and a homogeneous, defect-free protective coating of a metal phosphateor arsenate glass on at least the internal surface of the envelope andthe exposed surfaces of internal components other than incandescentportions of said filament which tend to react with said halogen duringoperation of the lamp.

2. A lamp according to claim 1 wherein said gaseous fill comprises afluorine-supplying material selected from elemental fluorine andfluorine-containing compounds.

3. A lamp according to claim l wherein said coating comprises a glassselected from phosphate and arsenate glasses of at least one of themetals aluminium. iron, chromium titanium, vanadium and tin.

4. A lamp according to claim 2 wherein said coating comprises analuminium titanium phosphate glass.

5. A lamp according to claim 3 wherein the atomic ratio of metal tophosphorus or arsenic in the glass composition is from l:0.l to l:2.9.

6. A lamp according to claim 1 wherein said coating is the deposited andbaked residue of a solution of a solvent-soluble complex phosphatecontaining coordinated solvent g-oups.

1. A TUNGSTEN-HALOGEN LAMP COMPRISING: A LIGHT-TRANSMITTING ENVELOPE, INTERNAL COMPONENTS INCLUDING A TUNGSTEN FILAMENT AND SUPPORTS THEREFOR SEALED WITHIN SAID ENVELOPE, ELECTRICAL LEADS FOR SAID FILAMENT SEALED INTO SAID ENVELOPE, A GASEOUS FILL INCLUDING HALOGEN IN SAID ENVELOPE, AND A HOMOGENEOUS, DEFECT-FREE PROTECTIVE COATING OF A METAL PHOSPHATE OR ARSENATE GLASS ON AT LEAST THE INTERNAL SURFACE OF THE ENVELOPE AND THE EXPOSED SURFACES OF INTERNAL COMPONENTS OTHER THAN INCANDESCENT PORTIONS OF SAID
 2. A lamp according to claim 1 wherein said gaseous fill comprises a fluorine-supplying material selected from elemental fluorine and fluorine-containing compounds.
 3. A lamp according to claim 1 wherein said coating comprises a glass selected from phosphate and arsenate glasses of at least one of the metals aluminium, iron, chromium titanium, vanadium and tin.
 4. A lamp according to claim 2 wherein said coating comprises an aluminium titanium phosphate glass.
 5. A lamp according to claim 3 wherein the atomic ratio of metal to phosphorus or arsenic in the glass composition is from 1:0.1 to 1:2.9.
 6. A lamp according to claim 1 wherein said coating is the deposited and baked residue of a solution of a solvent-soluble complex phosphate containing coordinated solvent groups. 